1. Field
Embodiments of the present invention generally relate to computer networking. In particular, embodiments of the present invention relate to selecting among multiple concurrently active paths through a network.
2. Description of the Related Art
Over the past several years, the computing and storage server industries have been migrating towards a network-based computing and storage model to take advantage of lower cost, high-performance commodity processors and lower cost, high-density storage media. This server industry trend has created a need for a highly scalable interconnect technology to enable the various computing and storage resources to be efficiently and effectively coupled. One type of interconnect that has been considered for this purpose is an Ethernet network.
An Ethernet network is a loop-free switching path, reverse path learning network. By “loop-free”, it is meant that there is only one path between any pair of nodes in the network. Because of this loop-free property, it is possible for the switches in an Ethernet network to forward packets by broadcast flooding, and to populate their forwarding tables through reverse path learning.
Specifically, when an Ethernet switch encounters a packet with a destination node address that it does not have in its forwarding tables, the switch broadcasts that packet on all outgoing links, except for the link on which the packet was received. All subsequent switches that receive the packet that do not have the destination node address in their forwarding tables do the same thing. Eventually, the packet will be delivered to the destination node. Because there is only one path to the destination node, it is assured that broadcasting the packet in this way will not create an infinite loop of broadcasts.
In addition to broadcasting the packet, a switch also determines, from the packet, the address of the source node that sent the packet. It also notes the link on which the packet was received. This address and link association is stored in the forwarding tables of the switch. In the future, if the switch receives any packet destined for the source node, it will know, based upon the address and link association in the forwarding tables, which link to switch the packet to. It will not need to broadcast the packet. In this way, an Ethernet switch learns the reverse path of a packet. Because of this reverse path learning capability, it is not necessary to pre-configure the forwarding tables of Ethernet switches. The switches can build these forwarding tables on the fly. This self learning capability of Ethernet switches is a key “plug and play” attribute of an Ethernet network, and is one of the reasons why Ethernet is so widely deployed.
While the loop-free aspect of an Ethernet network gives rise to certain advantages, it also is the root of several significant drawbacks. First, because there is only one path between each pair of nodes, the network does not recover from failure as quickly as would be desired. When a link in a path is disabled, another path has to be determined and deployed. This takes a relatively long time, and during that time, nodes coupled via that link cannot communicate. Another drawback is that the single path between each pair of nodes limits the cross section bandwidth of the network; thus, the switching capacity of the network is underutilized. Furthermore, because there is only one path between each pair of nodes, it is not possible to spread and balance the traffic across a plurality of paths. For these and other reasons, Ethernet, as it has been implemented, has not been an effective interconnect for coupling computing and storage resources in a network-based, high-performance system.